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AtomicTheory ClassicalParticle
Particle Based Atomic Theory History "Nothing exists except atoms and empty space; everything else is opinion." This was Democritus' opinion on the atoms that he and Leucippus defined to be small, indivisible particles that make up the rest of the world. Their work was the first theory of this kind and took place in the fifth century B.C. It was not accepted, however, until much later because Aristotle had an opposing view and was much more influential at that time. Because there was no known way to prove either way, Aristotle's view prevailed that everything was made up of fire, air, earth, and water. It was not until the 16th century when modern science began to emerge that Democritus' theory was seriously considered and was not until the early 1800s when John Dalton's work created sustantial proof for Democritus' theory that it began to be accepted. John Dalton created his atomic theory through use of the Law of Conservation of Mass, the Law of Definite Proportions, and the Law of Multiple Proportions. It states #Elements are composed of tiny, indistructable particles called atoms #All atoms of a given element are the same in mass and other distinguishing properties #Atoms combine in simple, whole-number proportions to form compounds #Atoms of one element cannot change into atoms of another element, they can only change what elements they are bounded to This Atomic theory is now widely accepted, supported and is the foundation for further research into the structure of the atom that is found in the Modern Atomic Theory. Modern Atomic Theory The modern Atomic theory is built up from a number of famous experiements and works to explain the structure of atoms. #In the late 1800s J. J. Thomson's Cathode Ray experiment proved the existance of the electron. #In 1909, Robert Millikan's Oil Drop experiment measured the charge of an electron. #Also in 1909, Ernest Rutherford used his Gold Foil experiment to explain the structure of an atom. #in 1932, James Chadwick discovered the neutron. #In 1915, Bohr proposed the Bohr Model of an atom which describes electron movement #In 1926, Erwin Schrodinger expanded upon Bohr's model to reach the modern picture of the atom Cathode Ray Experiment [Photograph of Cathode Ray Tube] A cathode ray tube is a device that shoots rays of electrons. Physicists were aware at this time in history that the ray carried a negative charge along with it but were unsure if the charge could be seperated from the ray or not. J.J. Thomson used a modified version of a Cathode Ray Tube and was able to prove that both a magnetic field and an electric field were capable of bending the ray so that no electrons or negative charge could be measured where they were anticipated to land. This proved that the negative charge is an inseperable trait of electrons, and that these negative electrons exist in atoms. see http://explorable.com/cathode-ray-experiment Oil Drop Experiment [Diagram of Oil Drop Experiment] The Oil Drop Experiment that Millikan performed was a complex experiment to determine the exact charge of an electron. He sprayed charged oil drops into a container and then used charged plates to determine how strong of an electric field was necessary to stop the oil drops from falling. Using the force of gravity and the mass of the oil drops he was then able to calculate the force of gravity on each drop and how much electric charge each drop must have. He discovered by varying the charge on the oil drops that the charge was always a multiple of -1.6x10^-19. This means not only that charge is quantized, but that the charge of a single electron must be -1.6x10^-19. see http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm also see http://regentsprep.org/Regents/physics/phys03/amillikan/ and see http://www.kentchemistry.com/links/AtomicStructure/Millikan.htm for a video explanation Gold Foil Experiment [Diagram of Gold Foil Setup] Rutherford's Gold Foil Experiment gave insight into the structure of an atom by proving that they are made up primarily by empty space. What he did was set up a piece of very thin gold foil and then shoot radioactive alpha particles through the foil. The entire system was surrounded by a circular sheet of zinc sulfide to detect where the alpha particles landed. If the atoms were solid, the particles would have all been deflected back in the direction that they came from. However, this was not at all the case. Nearly all of the alpha particles went straight through the foil and were detected on the other side with some particles deflected off in random directions. This means that most of an atom is empty space that can allow the alpha particles to pass through unaffected. Additionally, however, the particles that were deflected imply that there is a part of the atom dense enough to stop even alpha particles. These results are what lead to both the discovery of the nucleus of an atom and the electron cloud. Further study of the angle at which the particles were deflected would also lead to the discovery of the proton. Very few particles were deflected by more than 90 degrees. This would occur if the particles directly collided with the nucleus of an atom. More particles were deflected less than 90 degrees though. This would occur because the negative radioactive particles were being affected by a positive magnetic field and being skewed off course. The particles that were deflected in this way proved that the nucleus contains a positive charge, now known as a proton. see http://www.rsc.org/chemsoc/timeline/pages/1911.html also see http://chemed.chem.purdue.edu/genchem/history/proton.html Discovery of the Neutron It was known that the atomic mass was approximately twice the expected mass of protons in an atom, but until James Chadwick's work scientist were unsure of what made up this mass. It was assumed that there were electrons in the nucleus holding it together that made up the extra mass. Chadwick was able to prove that there are no electons in the nucleus and from there was able to conclude the existance of neutral particles in the nucleus, now know as neutrons. see http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrondis.html for the math that Chadwick used to support his theory. Bohr's Model of an Atom [Bohr Model Diagram] Bohr used the idea that Millikan discovered which states that energy of electrons is quantized and the discoveries of Rutherford and Chadwick to put together a diagram of the atom. His model consists of the nucleus of the atom as a dense collection of protons and neutrons in the center with electrons circling the nucleus in specific orbital levels. It is the jump between these orbitals that releases photons of energy and it is the fact that photons are only found with certain energies that proves this concept. Bohr's model works off of several new ideas that he proposed to be true. This include the idea that electrons orbit the nucleus without losing energy and can moved only in fixed orbits of specific energies. He also claimed that electrons with low energy would orbit in the orbitals closest to the nucleus and electrons with greater energy could jump to higher orbitals. see http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html for mathematical proofs. also see http://abyss.uoregon.edu/~js/glossary/bohr_atom.html Schrodinger's Model of an Atom [Schrodinger Model Diagram] The modern model of an atom has not changed much since Bohr's model. The main difference that Schrodinger discovered is the idea that electrons orbits are not exact, and are much more accurately described by the probability of where an electron might be found at any current moment. The patterns that arise are very similar to Bohr's model with enough snapshots of the electron depicted. From here, Schrodinger was able to conclude that, if described by probabilities, Bohr's idea of electron orbitals was correct. He went on to define the Quantum Model of an Atom. This model explains the shape of all of an atom's possible orbitals, how these orbitals interact during bonding, and more. see http://chemed.chem.purdue.edu/genchem/history/schrodinger.html and http://timelineoftheatom.blogspot.com/p/erwin-schrodinger-1926.html for an overview of entire history of the model of the atom and a more detailed explanation of the modern model, see http://profmokeur.ca/chemistry/history_of_the_atom.htm